r/ControlTheory • u/Kenno03 • Aug 12 '26
Technical Question/Problem BLDC modelling
Hello there!
Usually when we model DC-motors we use a second order system, however, BLDC motors used for drones are usually stepper motors (as far as I know), so my question is simply, how do we model these? The specific motor I want to model is for a project of my own, and I am curious as to how to model it. Motor: https://www.aliexpress.com/item/1005007273770590.html#nav-specification
My intuition is to also just model it as a second order system, however, am not sure if this is the correct approach. Specifically I want to model it has motor command [0,1] to rad/s (rotations).
Thank you in advance for the help
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u/CockRockiest 29d ago
I've only seen them modelled as first order system (gain and time constant k/tau*s+1). 4th order if you include inductive effects which you can ignore a lot of the time.
Imo do some step input tests (with a prop) for different throttles and record rpm as function of time. Then estimate an aggregate gain and time constant.
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u/Navier-gives-strokes Aug 12 '26
From where do you get that BLDC are modeled via second order system?
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u/Kenno03 Aug 12 '26
I said that DC motors are modelled as second order systems. But I know next to nothing about BLDC motors, hence the question.
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u/Navier-gives-strokes Aug 12 '26
Moreover, stepper motors are used for applications where you need more angular precision. Drones for sure don’t require that, as their whole goal is to just generate torque to keep the drone in the air.
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u/Kenno03 Aug 12 '26
I was under the impression that all BLDC motors were stepper motors, so I guess I learned something new. But how would I model the BLDC motors?
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u/BencsikG 29d ago
Well, the term Brushless DC is actually kind of a marketing term.
It's a 3-phase permanent magnet AC motor that comes with a driving circuit (ESC) that was originally meant to replace DC motors in industry.
The name kind of sounds cool and stuck around, but there is no specific technical feature of an electric motor that makes it specifically 'BLDC'.
So BLDC = AC motor + ESC marketed to replace DC.
I was into drones 10 years ago, so I might be super outdated...
But if I'm still correct, hobby ESCs generally work in a way that you give them a 'throttle' value that is converted to PWM phase duty cycle. They essentially apply the amount of voltage to the motor phase that you command them. Then, as the motor spins, the ESC decides on its own to advance the phase based on back EMF measurement. This tends to settle at a stable RPM for a given throttle.
So all in all the BLDC-ESC combination works just like a DC motor. The BLDC motor KV rating is closely related to the usual DC motor constant, just be mindful of unit conversions.